This invention is directed to an apparatus and method which is useful in wavelength division multiplexing systems. More particularly, this invention is directed to an optical apparatus having a predetermined group velocity dispersion of signals.
Wavelength division multiplexing (WDM) is a technique for increasing transmission capacity in fiber optic communication systems. To this end, a number of optical devices have been researched and developed including optical routers.
A typical optical router has at least one input port and at least one output port. Light associated with an optical signal is coupled from an input port to an output port according to the carrier wavelength of the optical signal. Examples of optical routers include multiplexers, demultiplexers, and Nxc3x97N optical routers.
A number of problems, however, must be overcome whenever a WDM system is used for high data rate transmission. These problems include group-velocity dispersion (GVD) and differential group delay (DGD).
GVD is one problem that arises when data rates are increased in a WDM system. If the magnitude of the group-velocity dispersion of the system is sufficiently large, the optical pulses that are transmitted as adjacent pulses will be received as pulses that overlap to a significant extent. The overlapping of adjacent pulses increases the bit error rate of the system and consequently degrades the performance of the fiber optic system. In order to prevent this performance degradation, typically all components of the system are required to have a value of group-velocity dispersion within a certain tolerance. The tolerance limits become smaller as the data rate increases for a particular application. For high bit rate applications the optical router is commonly required to have a value of group-velocity dispersion that is low or sufficiently close to zero throughout the band associated with each wavelength channel. The optical router must have a low value of the absolute value of the group-velocity dispersion within a substantial portion of the passband of each channel.
Differential group delay (DGD) is another problem that must be overcome when data rates are increased in a WDM system. DGD is the group delay for the polarization state that provides that largest group delay minus the group delay for the polarization state that provides the lowest group delay. In order to prevent a fiber optical transmission system from being degraded by polarization mode dispersion, each of the components must have sufficiently low DGD. In general, DGD is positively correlated with GVD. Therefore, ensuring that GVD is low typically ensures that DGD is low.
One technique for fabricating an optical wavelength router is planar lightwave circuit (PLC) technology. A typical PLC comprises planar waveguides and/or channel waveguides. Examples of planar and channel waveguides are shown in H. Kogelnik, Theory of Optical Waveguides, Guided-Wave Optoelectonics T. Tamir ed., Springer-Verlag, Berlin, 1988, and also by H. Nishihara, M. Haruna, and T. Suhara, Optical Integrated Circuits, McGraw Hill, New York, 1987.
In a planar (or slab) waveguide, light is generally restricted to propagate in a region that is thin (typically between 3 xcexcm and 30 xcexcm) in one dimension, referred to herein as the lateral dimension or height, and extended (typically between 1 mm and 100 mm) in the other two dimensions. Herein, we refer to the plane that is perpendicular to the lateral dimension of the PLC as the plane of the PLC. The longitudinal direction is defined to be the direction of propagation of light at any point on the PLC. Further, the lateral direction is defined to be perpendicular to the plane of the PLC and the transverse direction is defined to be perpendicular to both the longitudinal and the lateral directions.
In a channel waveguide, light has an optical field that is substantially confined in both the lateral direction and the transverse direction. In a typical channel waveguide, the field is substantially confined within a region that extends between 3 xcexcm and 30 xcexcm in the lateral direction, herein referred to as the height, and extends between 3 xcexcm and 100 xcexcm in the transverse direction, herein referred to as the width.
Typically, the optical field of light that propagates in a channel waveguide comprises a linear combination of normal modes. The normal modes may be denoted as Expq and Eypq, where p and q may be any non-negative integer and x and y are used to denote the polarization of the mode, x referring to the lateral direction and y referring to the transverse direction. See H. Nishihara, M. Haruna, and T. Suhara, Optical Integrated Circuits, McGraw Hill, New York, 1987, p. 29. Herein xcfx86i refers to either Ex0,ixe2x88x921, or Ey0,ixe2x88x921 or a linear combination of Ex0,ixe2x88x921 and Ey0,ixe2x88x921 as the case may be. That is, xcfx861 refers to a mode that has no nodes in either the lateral or the transverse directions and xcfx863 refers to a mode that has no nodes in the lateral direction and two nodes in the transverse direction. Herein the xcfx861 mode may be referred to as the fundamental mode or, alternatively to the first mode. Herein the xcfx863 mode may be referred to as the third mode.
There are various approaches to building a PLC. In a typical example of a PLC, a slab waveguide comprises three layers of silica glass with the core layer lying between the top cladding layer and the bottom cladding layer. Channel waveguides are often formed by at least partially removing (typically with an etching process) core material beyond the transverse limits of the channel waveguide and replacing it with at least one layer of side cladding material that has an index of refraction that is lower than that of the core material. The side cladding material is usually the same material as the top cladding material. Further, each layer may be doped in a manner such that the core layer has a higher index of refraction than either the top cladding or bottom cladding. When layers of silica glass are used for the optical layers, the layers are typically deposited on a silicon wafer. As a second example, slab waveguides and channel waveguides comprise three or more layers of InGaAsP and adjacent layers can have compositions with different percentages of the constituent elements In, P, Ga, and As. As a third example, one or more of the optical layers of the slab waveguide and/or channel waveguide may comprise an optically transparent polymer. A fourth example of a slab waveguide comprises a layer with a graded index such that the region of highest index of refraction is bounded by regions of lower indices of refraction. A doped-silica waveguide is usually preferred because it has a number of attractive properties including low cost, low loss, low birefringence, stability, and compatibility for coupling to fiber.
In addition to the channel and slab waveguides described above, various PLCs may comprise at least one optical dispersive region such as, for example, an arrayed waveguide. An arrayed-waveguide grating router (AWGR) is a planar lightwave circuit and comprises at least one input channel waveguide, an input slab waveguide, an arrayed-waveguide grating (AWG), an output slab waveguide, and at least one output channel waveguide. The edge of the input slab waveguide to which the input waveguides are attached is referred to herein as the input focal curve. The edge of the output slab waveguide to which the output waveguides are attached is referred to herein as the output focal curve.
The arrayed-waveguide grating comprises an array of waveguides. The length of the ith waveguide in the AWG is denoted as Li. The angular dispersion that is provided by the AWG is determined in part by the difference in length between adjacent waveguides, Li+1xe2x88x92Li. The details of construction and operation of the AWGR are described in M. K. Smit and C. Van Dam, PHASAR-Based WDM-Devices: Principles, Design, and Application, IEEE Journal of Selected Topics in Quantum Electronics, vol. 2, no. 2, pp. 236-250 (1996); K. McGreer, Arrayed Waveguide Gratings For Wavelength Routing, IEEE Communication Magazine, vol. 36, no. 12, pp. 62-68 (1998); and K. Okamoto, Fundamentals of Optical Waveguides, pp. 346-381, Academic Press, San Diego, Calif., USA (2000). Each of the publications and patents referred to in this application is herein incorporated by reference in its entirety.
One type of AWGR is a Gaussian-passband AWGR (G-AWGR). In a G-AWGR, the length difference between adjacent waveguides of the AWG, Li+1xe2x88x92Li, is substantially independent of I (i.e., Li+1xe2x88x92Li is substantially constant throughout the AWG.). This type of AWGR is described in K. Okamoto, Fundamentals of Optical Waveguides, pp. 346-360, Academic Press, San Diego, Calif., USA (2000). Herein, the coupling width is defined as the width that the input or output waveguide has at the point where it is attached to its respective slab waveguide. This construction generally results in an AWGR that provides passbands that are approximately Gaussian in shape.
The shape of the passband is determined by the convolution of two fields. The first field in the convolution is the field that is formed from the light that passes through the AWG and is imaged onto the output focal curve. The second field in the convolution is the fundamental mode of the output waveguide at the output focal curve. In the G-AWGR, both fields in the convolution are substantially Gaussian, and, consequently, the passband is substantially Gaussian. In an ideal G-AWGR there is no phase error and no amplitude error and, consequently, the group delay and the GVD are both equal to zero at the center of the passband. See H. Yamada, K. Okamoto, A. Kaneko, and A. Sugita, Dispersion Resulting From Phase And Amplitude Errors In Arrayed-Waveguide Grating Multiplexers-Demultiplexers, Optics Letters, vol. 25, no. 8, pp. 569-571 (2000). In practice, however, fabrication errors lead to phase and amplitude errors that may cause a non-zero GVD at the center of the passband.
Another type of AWGR is a passband-flattened AWGR (PF-AWGR). The passband of the PF-AWGR is typically broader than the passband of a G-AWGR. In this context, a passband that is relatively broad refers to a passband having a value of flatness that is relatively large wherein flatness is defined as the xe2x88x921 dB passband width divided by the xe2x88x9220 dB passband width. Typically, a G-AWGR has a passband flatness of approximately 0.22, and typically a PF-AWGR is required to have a flatness of 0.3 or larger. This is advantageous because many applications require the passband to be broader than can be provided by the G-AWGR.
There are a variety of techniques to broaden the passband of an AWGR. One technique for broadening the passband of an AWGR involves the introduction of an MMI coupler between the slab waveguide and the channel waveguide at either the input side or the output side. See, for example, J. B. D. Soole et al., Use Of Multimode Interference Couplers To Broaden The Passband Of Wavelength-Dispersive Integrated WDM Filters, IEEE Photonics Technology Letters, vol. 8, no. 10, pp. 1340-1342 (1996); U.S. Pat. No. 5,629,992 to Amersoot et al; and U.S. Pat. No. 5,412,744 to Dragone. Use of MMI couplers, however, results in excess optical insertion loss.
Another technique for broadening the passband of an AWGR involves the introduction of a xe2x80x9chornxe2x80x9d between the slab waveguide and the channel waveguide at either the input side or the output side. Employing a horn instead of the MMI coupler is advantageous because the horn provides a smoother and more adiabatic transition and because its utilization is the most tolerant to variations in the fabrication process. Adiabatic transitions are generally employed because they minimize transition loss.
An example of a PF-AWGR optical router 10 is depicted in FIG. 1A and is similar to that disclosed in K. Okamoto and A. Sugita, Flat Spectral Response Array-Waveguide Grating Multiplexer With Parabolic Waveguide Horns, Electronics Letters, vol. 32, no. 18, pp. 1661-1662 (1996). The router 10 includes one or more input waveguides 20, an input slab waveguide 30, an arrayed waveguide grating (AWG) 40, one output slab waveguide 50, and one or more output waveguides 60.
As shown in FIG. 1A, the input waveguides 20 are coupled to the input slab waveguide 30 via a parabolic waveguide horn or taper 80. Typically, the length of such a taper is between 150 xcexcm and 1500 xcexcm.
FIG. 1B depicts an expanded view of a parabolic waveguide horn or taper. The width of the taper is shown increasing gradually such that the width is largest at the input focal curve 90. In this example, the width of the taper as a function of the distance in the longitudinal direction is described by the parabolic function
W2=(z/L)(WL2xe2x88x92W02)+W02,xe2x80x83xe2x80x83(1) 
where WL is the maximum width of the taper, W0 is the minimum width of the taper, L is the length of the taper, z is the distance from the narrow end of the taper in the longitudinal direction, and W is the width of the taper at an arbitrary value of z. This shape of taper may be characterized as a parabolic taper.
Another taper design is shown in U.S. Pat. No. 6,069,990 to Okawa et al.
None of the above referenced designs, however, disclose an apparatus and method having a predetermined group velocity dispersion (GVD) as disclosed herein.
There is still a need for an optical apparatus that simultaneously broadens the passband and maintains low GVD.
There is also still a need for a useful method and apparatus incorporating a transitional segment as disclosed herein.
It is an aspect of the present invention to provide a method and apparatus that maintains the GVD within a predetermined range.
It is another aspect of the present invention to provide a method and apparatus to dynamically control the GVD in an optical apparatus.
It is still another aspect of the present invention to provide a method and apparatus to provide a controlled non-zero GVD in order to compensate for other non-zero GVD values introduced by other component.
Still other aspects and features of the present invention will become apparent in view of this disclosure.
The present invention is useful in optical communication systems. In particular, the present invention is capable of providing a selected group velocity dispersion of signals in wavelength division multiplexing systems.
The present invention optically couples a slab waveguide to at least one channel waveguide via a transition segment. The transition segment includes a taper and a taper extension. The taper has a narrow end and a wide end and the narrow end of the taper is configured to optically couple to said channel waveguide. The transition segment further includes an extension. The extension has a first end and a second end wherein the first end of the extension is configured to optically couple to the wide end of the taper and the second end of the extension is configured to optically couple to the slab waveguide and wherein the first end and the second end are equal in width.
One variation of the present invention is an optical apparatus which comprises at least one input waveguide, a first slab waveguide optically connected to the at least one input waveguide, a second slab waveguide optically connected to the first slab waveguide via an optical dispersive region, at least one output waveguide optically connected to the second slab waveguide, and at least one transition segment optically connecting the at least one input waveguide to the first slab waveguide wherein the at least one transition segment comprises a taper and an extension optically connecting a wide end of the taper to the first slab waveguide.
The present invention may include a plurality of transition segments and the transition segments may be positioned on the input side as described above, the output side of the device, or on both the input and the output side of the device. When a plurality of extensions are present, each transition segment may be identical or different. Properties such as width, length, curvature, angle, and material may be pre-selected.
The inventive transition segments may also include a heating element controlled by a programmable device such as a controller or computer. A feedback loop can be used to maintain a constant temperature at the extension. Alternatively, the transition segments may include an electro-element or an acousto-element.
In another variation of the present invention, the optical apparatus includes an arrayed waveguide grating. Alternatively, the optical apparatus may employ an integrated reflection grating.
In another variation of the present invention, the optical apparatus includes, in addition to one or more transition segments on the input side, at least one output taper optically connecting the at least one output waveguide to the second slab waveguide wherein the at least one output taper comprises a first end and a second end having a width wider than the first end and wherein the second end is optically connected to said the slab waveguide. The width of the at least one output taper can be, but is not limited to, sufficiently narrow such that optical guiding is provided in only the fundamental mode; sufficiently wide such that optical guiding is provided for the third mode and coupling from the fundamental mode into the third mode is substantially negligible; parabolic shaped; or linear shaped. Alternatively, the output side may feature a constant width waveguide and no taper.
Another variation of the present invention includes a method for improving performance of an optical communication system. The method comprises establishing a value of GVD in an optical component of the optical communication system such that performance is improved. The value of GVD is established by providing an optical component comprising at least one transition segment configured to optically couple at least one channel waveguide to a slab waveguide. The transition segment comprises a taper having a narrow end and a wide end with the narrow end of the taper configured to optically couple to the channel waveguide. The transition segment further comprises an extension having a first end and a second end wherein the first end of the extension is configured to optically couple to the wide end of the taper and the second end of the extension is configured to optically couple to the slab waveguide. The first end and the second end having an equal width.
A variation of the inventive method includes providing said optical apparatus with a plurality of extensions each having a different length to provide a non-zero GVD which cancels a non-zero GVD introduced from another optical communication component.
Other variations of the present invention will become apparent in view of the foregoing text and referenced drawings.